Solar-cell technology is currently poised to make significant progress in mass adoption due in part to the looming shortage of traditional energy sources, e.g. crude oil and natural gas, and to the increased awareness of “green-technology” benefits. Solar-cell technology, though capturing “free” energy from the sun, has been expensive with per-watt ownership cost ($/W) far exceeding $/W offered by electric utilities. Recently at $5/W, the pay-off period for a solar panel is as much as 50% of its lifespan, due largely to the expense of the semiconductor material used. The persistently high $/W figure has led to ideas on cost reduction, an example of which is concentrated photovoltaics (CPV). In CPV, the sun's energy is concentrated hundreds of times onto a solar cell. CPV, however, requires the solar cell to be very thermally robust due to high heat resulting from the concentration. Fortunately, the advent of new materials with more robust thermal properties and band gap energies better suited to the sun's spectrum has made solar technology attractive again through CPV.
Until recently, silicon (Si) has been at the core of solar-cell technology. However, efficiency for the best single-junction-Si-based cells only reaches about 22%. Recently, multi-junction solar cell designs have achieved efficiencies that far surpass single junction devices. In the multi-junction design, typically, each junction is formed of a different material. For example, III-V compound semiconductors (e.g., InGaAs, InGaP) and group IV materials (e.g., Ge), have been used together to make multi-junction solar cells. These multi-junction solar cells typically use a different material for each junction. Laboratory efficiencies as high as 40.7% in a three junction design using the three mentioned semiconductors have been claimed.
However, these III-V compound-semiconductor-based solar cells are more expensive than single junction Si devices due to material cost as well as manufacturing complexity. Therefore, these devices have been excluded in the traditional solar-panel business where sheer size requirement meant prohibitive cost. Space and other niche applications have, however, sustained the specialized interests in the more expensive but more efficient and robust solar cells based on the III-V compound semiconductors.
Therefore, while improvements have been made in solar cell efficiency, still further improvements in efficiency are desirable. It is further desirable that the per-watt ownership cost of solar cells be reduced.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.